Surface Characteristics of Polyhedral Oligomeric Silsesquioxane

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J. Phys. Chem. B 2008, 112, 11915–11922

11915

Surface Characteristics of Polyhedral Oligomeric Silsesquioxane Modified Clay and Its Application in Polymerization of Macrocyclic Polyester Oligomers Chaoying Wan,†,§ Feng Zhao,† Xujin Bao,*,† Bala Kandasubramanian,‡ and Matt Duggan‡ Institute of Polymer Technology & Materials Engineering, Loughborough UniVersity, Leicestershire, U.K., LE11 3TU, UK Materials Technology Research Institute, Pera InnoVation Park, Melton Mowbray, Leicestershire, U.K., LE13 0PB, and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, China, 200240 ReceiVed: June 15, 2008; ReVised Manuscript ReceiVed: July 21, 2008

Novel porous aminopropyllsooctyl polyhedral oligomeric silsesquioxane (POSS) modified montmorillonite clay complexes (POSS-Mts) with large interlayer distance and specific surface area have been successfully prepared via ion-exchange reaction and followed by freeze-drying treatment. The morphology of the POSSMts is highly influenced by the POSS concentration, pH of the suspension and drying procedure, but the interlayer distance of the POSS-Mts does not change much when the POSS concentration is above 0.4 CEC. The POSS-Mts were used as Sn-catalyst supporters to initiate the ring-opening polymerization of cyclic butylene terephthalate oligomers (CBT) for the first time. No diffraction peak was detected by wide-angle X-ray diffraction for the polymerized composites (pCBT/POSS-Mt), even at 10 wt % loading of POSS-Mt. A clay network rather than exfoliation structure was observed unexpectedly in the composites by transmission electron microscopy. The pCBT/POSS-Mt composite with 10 wt % POSS-Mt was further melt-compounded with commercial PBT resin as a master batch. The tensile properties of the resultant PBT/POSS-Mt composites were highly improved as compared to the pristine PBT due to the homogeneous dispersion of POSS-Mt in the PBT matrix. Introduction Polyesters like poly (butylene terephalate) (PBT) and poly (ethylene terephthalate) (PET) are important engineering polymers due to their excellent mechanical properties, solvent resistance and dimensional stability. Commercial polyesters have often been manufactured via condensation polymerization or solid-state polymerization.1 Recently preparation of polyesters from macrocyclic oligomers via ring-opening polymerization has become of interests.1,2 The butylene terephthalate or ethylene terephthalate cyclic oligomers are important macrocyclic polyester oligomers. Their unique properties such as low melt viscosity (0.017 Pa · s, water-like), rapid polymerization capability and no evolution of low-molecular-weight byproducts2b,c are especially proper for producing high performance polyester/ fibrous prepregs 2c,3 and polyester-clay composites.3 For the preparation of PBT- (PET-) clay nanocomposites, the low melt viscosity of macrocyclic oligomers allows it to swell or preintercalate organoclays, and form intercalated or exfoliated PBT- (PET-) organoclay nanocomposites after in situ ringopening polymerization associated by transesterification catalysts.3 To the best of our knowledge, the studies so far have chosen commercial organoclays as reinforcing agents, especially those modified by alkyl ammonium tallow salts. The alkyl ammonium tallow salts are good candidates for clay modification because of their long molecular chains and diversiform functional groups which can provide good compat* Corresponding author. Telephone: +44 (0) 1509 223150. Fax: +44 (0) 1509 223949. E-mail: [email protected]. † Institute of Polymer Technology & Materials Engineering, Loughborough University. ‡ UK Materials Technology Research Institute. § School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University.

ibility to polymers. However, their low thermal stability and potential of introduction of bioacitve contaminants to the final products highly limit their application in medical devices. Phosphonium,4a,b imidazolium,4c pyridinium,4d and ionic liquid4e have been developed for clay modification recently. These organic compounds improve the thermal stability of the organoclays to some extent, but they are not as good as ammonium tallow salts in expanding clay interlayers and improving the compatibility with polymers. In particular, the feasibility of these compounds in medical devices is still not clear. Polyhedral oligomeric silsesquioxane (POSS) is well-defined nanocluster with an inorganic silica-like core (Si8O12) surrounded by eight organic corner groups.5 It could be a good substitute for onium ions in clay modification because of its large molecular dimension, high thermal stability (thermal decomposition temperature is over 400 °C), biocompatibility, recyclability and nonflammability.6-9 POSS modified clays have been used as catalyst supporter for formation of polyethylene nanocomposites8 and exfoliated epoxy nanocomposites.9 But the colloid chemistry and texture structure of POSS intercalated clays are lacking of investigation, which limits its application in many fields. In this paper, aminopropyllsooctyl polyhedral oligomeric silsesquioxane modified montmorillonite clay complexes (POSS-Mts) are synthesized via ion-exchange reaction and followed by freeze-drying treatment. The surface characteristics of the POSS-Mts are investigated in terms of pH of suspension, POSS concentration and drying procedure in order to produce a new material for catalyst supporter or reinforcing agent for high performance polymer-clay nanocomposites. Macrocyclic butylene terephalate oligomer (CBT) is used as a precursor because of its low melt viscosity and rapid polymerization capability. The POSS-Mt supported Sn-based catalyst is prepared for the ring-opening polymerization of CBT. The

10.1021/jp805259q CCC: $40.75  2008 American Chemical Society Published on Web 08/30/2008

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effectiveness of the POSS-Mt-Sn in the polymerization of CBT is discussed based on the morphology development and tensile properties of the composites. The resultant PBT/POSS-Mt composites are expecting for medical device applications. Experimental Section Materials. Sodium montmorillonite clay (Closite Na, cation exchange capacity (CEC))92 mequiv/100 g, donated as NaMt) was purchased from Southern Clay Products Inc., Gonzales, TX. Aminopropyllsooctyl polyhedral oligomeric silsesquioxane (POSS) was purchased from Hybrid Plastic Co. Inc., USA. Butylchlorotin dihydroxide (Fastcat 4101, MW ) 245.29 g/mol, CH3(CH2)3Sn(OH)2Cl,) was purchased from Aldrich Chemical Co.. Cyclic butylene terephthalate oligomers (CBT, MW ) 220n (with n ) 2-7)) were obtained from Cyclics Corporation. Acetic acid, ethanol, and ammonia were purchased from Aldrich Chemical Co.. Polybutylene terephthalate (PBT) resin was supplied by Ticona Co., Ltd., UK, with the trade name of Celanex 2002-2 (MFI 250 °C/2.16 kg ) 20 cm3/10 min). Synthesis of POSS-Mt Complexes. First, the POSS was protonized by mixing with the same molar ratio of acetic acid at room temperature. Then 5 g of Na-Mt was dispersed in 250 mL of ethanol/deionized water (1:1, v/v) solution and kept stirring at 60 °C for 6 h. Then 5-10 mL of protonized POSS/ acetic acid/ethanol solution was introduced dropwise. The mixed suspension was stirred at 60 °C for 6 h, in which the POSS concentration was kept as 0.2, 0.4, 0.6, 0.8, and 1.0 times of cation exchange capacity (CEC) of Na-Mt, respectively. The resultant suspension was subjected to freeze-drying using a VirTis Benchtop SLC freeze-dryer (The Virtis Company Inc., Gardiner, NY) to produce POSS-Mt(x) with different POSS concentrations, the (x) representing the POSS concentration. In comparison, the POSS-Mt (x) was also produced by thermaldrying under air-flow at 80 °C for 24 h. Preparation of POSS-Mt Supported Catalyst. POSS-Mts (0.4 CEC)-supported butylchlorotin dihydroxide were prepared by an impregnation method as follows: 5 g POSS-Mts (0.4 CEC) were mixed with butylchlorotin dihydroxide/anhydrous ethanol solution and refluxed for 6 h. The resultant mixture was freezedried, leading to POSS-Mt-Sn catalysts. In Situ Ring-Opening Polymerization of CBT/POSS-Mt. The required amounts of CBT and POSS-Mt-Sn were mixed at 145 °C for 10min, then heated up to 200 °C to finish the ringopening polymerization reaction. The resultant composites were donated as pCBT/POSS-Mt composites. All the reactions were conducted under N2 at normal atmosphere. The POSS-Mt loadings were designed as 1, 2, 3, 5, and 10 wt %, respectively. The catalyst concentration was 0.5 wt % of CBT. Pure CBT was also polymerized under the same condition for comparison. Melt-Compounding of PBT with pCBT/POSS-Mt Master Batch. The pCBT/POSS-Mt composites (10 wt %) were meltcompounded with a commercial PBT in an internal mixer (HAAKE Rheocord 9000, Haake Co., Vreden, Germany) at 240 °C and 60 rpm for 15min to produce PBT/POSS-Mt composite, in which the POSS-Mt content was diluted to 2 wt %. For comparison, required amounts of pCBT and pCBT plus POSSMt (0.4 CEC) were directly melt-compounded with PBT under the same condition. Characterization and Testing. The microstructures of the POSS-Mts with and without catalyst supporting were characterized by X-ray diffraction (XRD, Siemens D5000), field emission scanning electron microscopy (FESEM, JEO 1530VP), energy dispersive X-ray analysis (EDX, EDAX phoenix), transmission electronmicroscopy (TEM, JEOL JEM-2100F), and thermo-

Figure 1. XRD patterns of Na-Mt, POSS, and POSS-Mts (0.4 CEC) suspensions in different pH values.

Figure 2. Rheological behavior of Na-Mt and POSS-Mt (0.4 CEC) suspension in different pH values.

gravimetric analysis (TGA, TA 2850). Static contact angle measurements for water drops on clay disks were performed using a dataphysics OCA20 contact angle equipment (Dataphysics Instrument GmbH, Germany) via a sessile drop method. Measurements made at five different points on each sample surface were averaged. Specific surface area was measured using nitrogen in gas sorption experiment (Coulter Omnisorp 100 CX, Beckman Coulter Inc.) at the temperature of liquid nitrogen and calculated by the Brunauer-Emmett-Teller (BET) equation using the data in a p/p0 range of 0.05∼0.4. Viscosity of clay suspension was measured by a rheometer (Rheolab QC, ANTON PAAR, Germany). The molecular weight of the synthesized pCBT with and without clays were analyzed using gel permeationchromatography(GPC,Modularunits)with1,1,1,3,3,3,hexafluoro-2-propanol as the solvent at 40 °C. The pCBT/POSSMt sample were thoroughly dissolved and filtered through 0.45 µm PTFE membrane into separate autosampler vials prior to chromatography. Tensile tests were carried out using an Instron universal material testing system (model 4465) according to ASTM D-638. Results and Discussion Surface Characteristics of POSS-Mt Complexes. Na-Mt belongs to dioctahedral smectite, whose surface consists of

Oligomeric Silsesquioxane Modified Clay

J. Phys. Chem. B, Vol. 112, No. 38, 2008 11917

Figure 4. XRD patterns of POSS-Mt (0.4 CEC) suspension in different pH values and dry POSS-Mts (0.4 CEC) powders.

Figure 3. TEM image for POSS-Mt (0.4 CEC) suspension in different pH: (a) pH)4.5, (b) pH)9.8.

planar siloxane surface and edge surface. The planar surface possesses net negative charges owing to isomorphous substitutions in the crystal lattice and the edge surface is usually associated with defect sites and pH-dependent charges. Normally the Na-Mt platelets flocculate into a three-dimensional, voluminous card-house structure in suspension due to the electrical interactions.10 The influence of pH on the morphology of POSSMt (0.4 CEC) suspension are shown in Figure 1. The XRD patterns of dry Na-Mt powder and pure POSS are also presented in Figure 1 for comparison. Dry Na-Mt exhibits a low-intensity

reflection at 2θ angle of 9.2°, corresponding to a d-spacing of 0.96 nm as calculated by Bragg’s equation. The POSS itself shows a typical diffraction peak at 7.2°, which belongs to the hexagonal crystalline structure.11 The Na-Mt is fully delaminated in ethanol/water suspension at pH ) 11.5 which is the pristine pH value of Na-Mt suspension, as indicated by its nonpeak pattern. Addition of POSS/acetic acid/ethanol solution into the Na-Mt suspension decreases the pH value from 11.5 to 4.5, and three diffraction peaks appear at 2.53°, 5.17° and 7.58°, suggesting that the delaminated Na-Mt layers reorganize into flocculation-intercalation structure during the ion-exchange process. The three diffraction peaks become smaller as pH value increases, and even disappear at pH)9.8. The absence of Bragg diffraction peaks indicates that the POSS-Mt loses their ordered packing morphology and redelaminates. The changes in POSS-Mt morphology in different suspensions can be reflected by the viscosity. As shown in Figure 2, all the POSS-Mt suspensions exhibit non-Newtonian flow behavior and the viscosity of the suspensions increases dramatically as the pH value increases. As the addition of POSS/acetic acid/ethanol solution into the Na-Mt suspension reduces the pH value from 11.5 to 4.5, the excess of protons create positive charges on the clay edges. As a result, the POSS-Mt layers associate into edge-to-face contacts due to heterocoagulation between the positive edges and the negative faces of the silicate layers. However, as ammonia is added dropwise into the POSS-Mt suspension to increase the pH, the edge-to-face association delaminates gradually due to the counteraction effect between the clay surface and the ammonia. The gradually delaminated POSS-Mts increase the amounts per unit volume of clay platelets, leading to an increase of the viscosity of the suspension. The different viscosity between Na-Mt (pH ) 11.5) and POSS-Mt (pH ) 9.8) suspension in which silicate layers are all delaminated is due to the addition of viscous POSS. The dispersion of POSS-Mt (0.4 CEC) in suspension can be clearly observed by TEM. From TEM images showed in Figure 3a, several POSS-Mt layers are partially packed, combining with folded layers. In comparison, the POSS-Mt is fully exfoliated into single layer with folded edges (Figure 3b). Since the surface of Na-Mt (>10000 nm2) is much larger than the POSS cage size (∼1.5 nm),6,11 the POSS can not be observed in the current enlargement scale. According to Figure 1, the POSS-Mt (0.4 CEC) might have been fully delaminated in the suspension (pH ) 9.8). If this

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Figure 5. Representative FESEM images combined with water-droplet contact angle images for POSS-Mts samples: (a) 0.4CEC, pH ) 4.5, freeze-drying; (b) 0.4CEC, pH ) 9.8, freeze-drying 0.6 CEC;(c) 0.4 CEC, pH ) 4.5, thermal-drying; (d) 0.6 CEC, pH ) 4.5, freeze-drying; (e) 0.8CEC, pH ) 4.5, freeze-drying.

morphology could be perserved after recovering of POSS-Mt out of suspension, it is highly benefical for the preparation of exfoliated polymer-clay nanocomposites. The POSS-Mt (0.4 CEC) suspension with pH ) 4.5 or pH ) 9.8 was subjected to freeze-drying and thermal-drying procedure, respectively. The structure and morphology of the POSS-Mt (0.4 CEC) suspension before and after drying treatment were characterized by XRD (Figure 4) and FESEM (Figure 5). From Figure 4, the dried POSS-Mt (0.4 CEC) demonstrates three diffraction peaks which is independent of the drying procedure and pH value. The freeze-drying method is supposed to preserve the original skeleton structure of clay hydrogels by substituting the liquid media for gas. While thermal-drying can collapse the structure

of clay hydrogels due to the surface tension effect caused by the evaporation of water.10 The POSS-Mts obtained from the two drying procedures exhibit similar XRD patterns (Figure 4), while the textural properties and physicochemical properties are different based on the FESEM observation (Figure 5)and BET measurement. Figure 5a shows that the freeze-dried POSS-Mts (0.4 CEC, pH ) 4.5) loosely pack and form edge-to-edge and edge-toface structure. Such card-house structure becomes rougher for the POSS-Mt recovered from the suspension (pH ) 9.8) as shown in Figure 5b. The thermal-dried POSS-Mt (0.4 CEC, pH ) 4.5) in Figure 5c are large and tightly packed aggregates, which was totally different from the freeze-dried clay in Figure

Oligomeric Silsesquioxane Modified Clay

J. Phys. Chem. B, Vol. 112, No. 38, 2008 11919 SCHEME 1: (a) Intercalation Process of POSS in Clay Galleries and (b) the Formation Mechanism of Card-House Structure of POSS-Mts

Figure 6. XRD patterns of Na-Mt, POSS, and freeze-dried POSSMts with different POSS concentrations.

5a. This clay tactoids with face-to-face aggregation are not easy to be redelaminated by macromolecules. The effect of POSS concentration on the basal spacing of Na-Mt is characterized by XRD (Figure 6). For the POSS-Mt (0.2CEC) sample, the characteristic diffraction peak of Na-Mt shifts to 7.1° (1.25nm), indicating the intercalation of POSS into the clay gallaries and subsequent expansion of the interlayer distance. When the POSS concentration increases from 0.4 CEC to 1.0 CEC, one high-intensity peak together with two smaller interference peaks emerge for the four POSS-Mts (0.4∼1.0 CEC), implying a high degree of regularity for the POSS modified clays. The high-intensity peak at 2.53° corresponding to a basal spacing of 3.58 nm suggests that the POSS molecules arrange in bilayer in the clay gallaries as the average size of Si-O cage structure of POSS is about 1.5 nm.6,11 The second basal spacing may be attributed to monolayer arrangement of POSS molecules within the clay layers. The montmorillonite clay allows much more molecules to intercalate into the interlayer space due to its expandable properties. The interlayer distance of the modified clays should increase with surfactant concentration increasing until the surfactant concentration reaches to a saturation limit, which is often equal to or above 1.0 CEC.12 Besides the surfactant concentration, the surfactant dimensions as well as packing density determine the morphology of modified clays.12,13 As shown in Table 1, the POSS-Mt decomposes in three steps within the temperature range of 200-600 °C. The onset decomposition temperatures are designed as T1, T2, and T3, respectively. The T1 should be ascribed to the decomposition of nonbonded surfactants on the clay surface. The T2 and T3 are caused by the decomposition of intercalated surfactants and

dehydroxylation of clays,14 respectively. The weight loss, exchange ratio and surface coverage of POSS-Mts increase with POSS concentration increasing from 0.2 CEC to 0.6 CEC and remain at around 45% of exchange rate and 81% of surface coverage for the POSS-Mts (0.6-1.0 CEC). These indicate that the POSS molecules just fill in the space by exchanging with sodium ions without further expanding the interlayer space when the POSS concentration is above 0.6 CEC. The cubic shaped POSS cannot be arranged as flexibly as other chain-like surfactants due to the steric hindrance effect. Once a double layer of POSS molecules assemble, the interlayer distance of the intercalated clay is almost defined. The d-spacing is expanded a little when the POSS concentration reaches to 1.0 CEC, indicating excess POSS molecules physically absorb on the clay surface. The corresponding intercalation process is illustrated in Scheme 1a. With the increase of POSS concentration from 0.4 CEC to 1.0 CEC, the d-spacing of POSS-Mts are in the range of 3.58∼3.87 nm, meanwhile, the card-house structure becomes rougher and the pore size among the packed clay layers increases as well (Figure 5d, 5e). The static contact angles are 103.3°, 108.7°, and 127.5° for freeze-dried POSS-Mt (0.4 CEC), POSSMt (0.6 CEC), and POSS-Mt (0.8 CEC), respectively. In comparison, the thermal-dried POSS-Mt (0.4 CEC) exhibits large and tightly packed clay aggregates with contact angle of 102.5°. The different contact angles in the samples reflects the different surface roughness and properties. The surface coverage data in Table 1 means about 19% of high-energy surface is

TABLE 1: Composition and Surface Coverage for Freeze-Dried POSS-Clay with Different POSS Concentration as Calculated from TGA Results from 200 to 600 oC POSS concentration 0.2CEC 0.4CEC 0.6CEC 0.8CEC 1.0CEC

T1 (°C)

T2 (°C)

T3 (°C)

weight loss (wt %)a

exchange ratio (%)b

surface coverage (%)c

276.9 259.4 280.5 273.6

347.7 367.8 353.7 373.7 341.1

534.3 403 537.5 403.2 532.6

7.6 23.8 33.5 32.2 33.6

7.4 28.9 45.6 44.6 46.8

13.5 51.0 82.3 77.6 82.7

a Weight loss in the range of 200∼600 °C. b Exchange ratio of POSS cation to sodium cation calculated according to the cation exchange capacity of the silicate (92 mequiv/100 g). c Surface coverage calculated based on the specific area of Na-Mt (750 m/g) and the particle size of POSS provided by its MSDS.

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Figure 7. FESEM images and EDX results of freeze-dried POSS-Mt (0.4 CEC, pH ) 9.8) before and after supporting Sn-catalyst.

uncovered, thus induces POSS-Mt platelets to form card-house structure through electrostatic interactions. The measured specific surface area of Na-Mt, freeze-dried POSS-Mt (0.4 CEC, pH ) 9.8) and thermal-dried POSS-Mt (0.4 CEC, pH ) 9.8) is 16.5, 136.8, and 118.6 m2/g, respectively. As shown in Figure 5, mesopores and micropores have formed in the freeze-dried samples obviously, and they become even larger as POSS concentration increases. The formation mechanism of the porous POSS-Mts is shown in Scheme 1b. The large specfic surface area and porous texture of POSS-Mts make it possible to be used as catalyst supporters or sorbents. The POSS-Mt (0.4 CEC, pH ) 9.8) supported Sn-catalyst for the ring-opening polymerization of CBT was investigated as follows. In Situ Ring-Opening Polymerization of CBT/POSS-Mts. Figure 7 shows the morphology of POSS-Mt (0.4 CEC, pH ) 9.8) before and after supporting Sn-catalyst (butylchlorotin dihydroxide). It is found that the card-house structure still preserves after impregnation treatment, and the Sn-catalyst is detected on the clay surface by EDX (Figure 7b). For in situ polymerization of polymer/clay nanocomposites, the preswelling of clay by monomer or oligomer is beneficial for the clay exfoliation. The low melt-viscosity of CBT (0.017 Pa · s) promotes the diffusion of CBT oligomers into the POSSMt intergalleries. In order to elucidate the reaction mechanism, two polymerization procedures for CBT/POSS-Mt polymerization were carried out. One-step polymerization is conducted by premixing of CBT, POSS-Mt (0.4 CEC, pH ) 9.8) and Sncatalyst together, and polymerizing subsequently. Two-step

polymerization means supporting POSS-Mt (0.4 CEC, pH ) 9.8) with Sn-catalyst first, which is then used to initiate the ringopening polymerization of CBT. The resultant pCBT/POSSMt composites from both polymerization procedures were characterized by WAXD. As shown in Figure 8a, the diffraction peaks of POSS-Mt were not observed after mixing with CBT and Sn-catalyst at 145 °C, indicating that the POSS-Mt was delaminated or fully swollen by the melted CBT. The diffraction peaks of POSS-Mt appeared again upon polymerization. Obviously, the delaminated or swollen POSS-Mt reorganized into their previous structure during the polymerization process. The position of diffraction peaks of POSS-Mt did not move but became much weaker after polymerization compared to those pristine peaks, suggesting that the clay layers were not further expanded but disorderly dispersed after polymerization.15 In Figure 8b, the absence of Bragg diffraction peaks suggests that the POSS-Mt might have been exfoliated or there are no areas in the sample in which the scattering length density fluctuations are periodic.16 The random dispersion of clays with no Bragg reflections were also found by Vaia et al.17 and Kurian et al.18 The dispersion condition of POSS-Mt was demonstrated by TEM and shown in Figure 9. In Figure 9a, POSS-Mt aggregates were observed in the composite prepared via the onestep polymerization. Unexpectedly, the POSS-Mt kept the cardhouse morphology rather than exfoliation in the composites prepared by the two-step polymerization (Figure 9b). One possible reason is that the POSS-Mt was fully swollen by melted CBT before polymerization, but the strong electrostatic interactions among the partially covered POSS-Mt’s surfaces and the

Oligomeric Silsesquioxane Modified Clay

Figure 8. WAXD patterns of pCBT/POSS-Mt (0.4 CEC) composites synthesized via (a) one-step polymerization and (b) two-step polymerization.

poor compatibility between POSS-Mt and pCBT chains induce the reorganization of the platelets. On the other hand, the adsorption or intercalation of POSS molecules on the clay surface influence the edge-to-edge and face-to-edge interactions of the clay crystallites, resulting in a higher degree of disorganization of the clay crystallites, which may cause the absence of the Bragg reflections. Additionally, the polymerization temperature for CBT/POSSMt-Sn system (two-step) is 175 °C, which is 20 °C lower than that of CBT/POSS-Mt/Sn system (one-step). The much lower

J. Phys. Chem. B, Vol. 112, No. 38, 2008 11921 polymerization temperature for the two-step system suggests that the POSS-Mt-Sn plays a cocatalytic role in the polymerization reaction due to the Lewis acidic sites on the clay surface. The Sn-catalyst anchoring on the POSS-Mt’s surface or residing in the pores of the POSS-Mt can speed up the intergallery reaction relative to that of bulk polymer. However, in the onestep polymerization process, the Sn-catalyst is mixed with POSS-Mt and CBT directly. The bulk polymerization of CBT is very fast and can be finished within several minutes. The sharply increased viscosity of the bulk polymer inhibits the POSS-Mt dispersion. That is why the POSS-Mt exhibits similar diffraction patterns before and after the one-step polymerization reaction. In addition, the molecular weight (Mw) of synthesized pCBT and pCBT/POSS-Mt-Sn (10 wt %, or 3.85 vol %) is 72 900 and 48 600, respectively. The presence of clay layers act as barriers for the chain propagation reactions although the Lewis acidic sites on the clay surface play a cocatalytic role in the chain initiation reactions. Interestingly, no diffraction peaks could be detected in the pCBT/POSS-Mt-Sn composites (two-step) even when the POSSMt content was as high as 10 wt % (Figure 8b). The pCBT/ POSS-Mt-Sn (10 wt % or 3.85 vol %) composite was used as a mater batch and melt-compounded with commercial PBT resin to dilute the POSS-Mt concentration to 2 wt % (0.77 vol %). As shown in Figure 8b, the PBT/POSS-Mt composite still does not show any obvious diffraction peaks within 1-10°. In Figure 9c, the POSS-Mt disperses uniformly as compared to those in Figure 9a and 9b. The card-house structure has been broken up during the melt-compounding process. So far, the low viscosity of CBT, facial polymerization (20 °C lower polymerization temperature), and disordered dispersion of POSS-Mt (the absence of Bragg reflections in WAXD) make it suitable for the pCBT/POSS-Mt-Sn composite for master batch application. The tensile properties of pure PBT/pCBT and PBT/POSS-Mt (2 wt %, master batch method) composite, as well as PBT/POSS-Mt (2 wt %, add directly) composite were investigated. The results are shown in Figure 10. It is found that the tensile strength and Young’s modulus of PBT/pCBT was enhanced up to 25% and 30% for the PBT/POSS-Mt (2 wt %, master batch method) composite, respectively, while they were 13% and 13% increase for the PBT/POSS-Mt (2 wt %, add directly) composite. The POSS-Mt remained card-house structure after in situ polymerization reaction, but it could be dispersed uniformly after further melt-compounding process. The better dispersion condition of POSS-Mt in the PBT/POSS-

Figure 9. TEM images obtained for pCBT/POSS-Mt composites with 2 wt % of POSS-Mts prepared via (a) one-step polymerization, (b) two-step polymerization, and (c) melt-compounding by letting down from a master batch (10 wt %).

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Wan et al. for medical devices due to the biocompatible and thermal stability of POSS. Acknowledgment. The authors gratefully acknowledge the financial support of the UK Department of Trade and Industry/ Technology Stategy Board, and the technical support from researchers and technicians of Pera Innovation Park, Loughborough University, Rondol Technology, Advanced Surface Polymers Ltd, AKI Ltd. and Select Moulds Ltd., U.K. References and Notes

Figure 10. Tensile properties of PBT and PBT/POSS-Mt composites prepared from different methods.

Mt composite (master batch method) resulted in the higher tensile properties than the composite prepared by directly meltcompounding. Conclusion Novel porous POSS modified montmorillonite clay complexes (POSS-Mts) with large interlayer distance and specific surface area have been successfully prepared via ion-exchange reaction and followed by freeze-drying procedure. The morphology of the POSS-Mts is highly influenced by the POSS concentration, suspension acidity and drying procedure, but the interlayer distance of the POSS-Mts does not change much when the POSS concentration is above 0.4 CEC. The partial surface coverage of POSS on the clay surface leaves high-energy surface uncovered, resulting in formation of card-house structure. Freeze-drying method can perserve this card-house structure resulting in porous POSS-Mt complexes. The special texture and properties of the POSS-Mt was used as catalyst supporter for CBT polymerization reaction. The POSS-Mt supported Sncatalyst (POSS-Mt-Sn) is fully delaminated by the melted CBT oligomers, but it reorganizes into ordered structure after polymerization. However, it can be redelaminated after meltcompounding with commercial PBT resin when the pCBT/ POSS-Mt-Sn composites (10 wt %) is used as a master batch. The tensile properties of PBT is highly improved even at 2 wt % of POSS-Mt. In addition, the polymerization temperature for CBT/POSS-Mt-Sn is 20 °C lower than that of CBT/POSS-Mt/ Sn system, indicating a cocatalyst effect of POSS-Mt on the CBT polymerization. In summary, the POSS-Mt supported catalyst is able to realize initiation and reinforcement of CBT at the same time, and the resultant nanocomposites are suitable

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